The show's gimmick begins with a renegade humanoid who asks to be called "The Doctor". He was born of an ancient race so advanced they have mastered both multiple lifespans (up to 13), and spatial and temporal dimensional engineering. They call themselves "Time Lords" and live on a peaceful and powerful planet called "Gallifrey".

(The show kept the Doctor's real name a teasy secret, like Lt Columbo's wife. The closest we humans can understand it is a huge math formula. So his childhood friends called him "Theta Sigma" for short.)

The Time Lord's main legal tenet is that they never use their immense powers of temporal engineering to accomplish anything at all. They have a zero-tolerance PrimeDirective.

The Doctor rebels against their unjust tyranny masquerading as passiveness, and steals a broken-down vehicle called a TARDIS (Time And Relative Dimensions In Space). This machine's cloaking device becomes permanently stuck on the setting "mid twentieth century Earth British Police Call Box", the British Invasion version of an emergency telephone station. This provides for amazing humor when passers-by, sometimes British and sometimes alien, accidentally witness it appear or disappear. (Most notably, the JohnCleese cameo in City of Death.)
TARDIS also makes an appearance as a jocular random encounter in the role-playing game "Fallout".

The Doctor teleports this device to random points in space and time (which often seem to suspiciously center on Earth or earthling colonies in our galaxy). He acquires a series of companions - humans, humanoids and robots - who help him unravel intricate gothic Shaggy Dog Tale mysteries at each of his more troublesome destinations. To keep the "SpaceOpera" fresh (and to re-invent its peculiar camp humor via broad cycles), every few seasons The Doctor metamorphoses into a new lead actor with a new set of characteristics & traits (see below for more information on regeneration).

The show-stopper episode called "The Pirate Planet" was the peak of the series's dizzying camp heights and of the reign of Tom Baker as the Doctor with the bushy hair, London Fog coat and 16-yard scarf. The show's script editor, DouglasAdams, wrote it.

At the time of writing (April 2005), Doctor Who is back on TV again with an updated budget, top actors and recognized script writers. It has been a great success in the UK and is definitely a series to look out for. See: http://www.bbc.co.uk/doctorwho/

Big Finish Productions do very good audio dramas starring the fifth, sixth, seventh and eighth Doctors and many original companions. They're a long way from the TV series, and one of the writers will be writing for the new series. Their web site is http://www.bigfinish.com

The names of many Dr. Who companions appear today among the children of the show's fans. Other Doctor Who memorabilia can be found all over the programming industry, the internet, and popular media; including SystemMetaphor names, NamesGivenToComputers ("tardis"), and the episode of TheSimpsons? where Sideshow Bob tried to eradicate television.

Also, in early 2002: IanMcKellen? portrayed a fan cosplaying as the Tom Baker Doctor, in one of the skits during his appearance on SaturdayNightLive?.

Trivia Question: Name an actor who appeared in both Dr. Who and StarWars Episode IV. What was his ominous line?

"Stay on target!"

Police Boxes were there for a number of reasons. The police used to store things in them, and used to lock arrested suspects in them while they called for assistance to take them back to the station. This is from before the days of radio communications. Even in 1963 they were common street furniture. (The copyright on the actual design was, I believe, bought by the BBC eventually...)

7 Tardis-style police boxes can still (Dec 2013) be seen in Glasgow, although most of them are red. There are at least two in London, although one is well hidden inside a police training ground.

The phone on the front is also for the use of the public, at a time when very few people had a telephone, and hence couldn't call 999.

The regeneration concept was invented when Hartnell decided to leave the show due to ill-health in 1966. Although he wanted to stay with the series, his health was failing (he died of heart disease in 1975). The word "regeneration" was first used at the time of the Pertwee/Baker transition.

The "Time Lord" backstory was only invented fairly late on in the show. The first two incarnations didn't have this backdrop, and it was sort of retrofitted. The doctor came across some beings who were kidnapping soldiers from various eras of history and setting them to fight against each other. Returning them all to their correct times was beyond his abilities, so reluctantly he had to call for help... from his hitherto unknown home planet. Unfortunately, this also revealed his presence, and they put him on trial for interference. He was sentenced to be exiled to his favourite planet, Earth. This conveniently cut the shooting budget at the time the series switched from black and white to colour filming.

Possibly the greatest tragedy of Doctor Who is that the early stories are missing many episodes. In the 70s, the BBC decided to rationalize its massive archive of recorded material. Since this was before the advent of home videotape rental or sales, they had no belief they would need many series. The telecine films of early episodes were destroyed and the 2in videotape recordings were wiped and the tapes recycled.

A massive effort by fans and TV historians has recovered many episodes from various sources - private collections, foreign TV stations who never returned their broadcast films, material that employees "liberated" from the destruction etc. However a large amount of the black and white material has never been found and is now unlikely to be recovered. So much material was lost that, until an episode was recovered in 2003, one of the Doctor's alien adversaries was known from a single photograph.